This invention relates to a computer assisted surgery system with enhanced graphics capabilities for use in inserting multiple screws into a hip fracture and orienting a total hip acetabular component.
Fractures of the femoral neck, one kind of hip fracture, are a common injury of the elderly with over 150,000 such fractures occurring annually. The currently accepted treatment for mildly displaced femoral neck fractures is surgical fixation. In the procedure, three cannulated screws are inserted in the hip, from the lateral aspect of the femur, across the fracture site and into the femoral head. The initial step is the placement of three guide pins, over which the screws are inserted, in a characteristic triangular pattern. This is typically performed under fluoroscopic guidance, usually with the aid of a drill guide. Some surgeons prefer to use a drill guide with a single bore, but doing so may make it difficult to achieve the desired triangular spacing and optimum placement of the guide pins in the femoral neck. Other surgeons prefer a drill guide with multiple bores that are fixed in the proper triangular spacing, but this requires a larger incision and doesn't permit the surgeon much flexibility to modify the preset pattern. With either drill guide, it can be difficult to accurately predict the final guide pin placement. Further, the surgery requires a significant number of fluoroscopic images and x-ray exposure for the surgeon, patient and operating room staff.
Fluoroscopic based image guided surgery systems can be of benefit in predicting the paths of the guide pins and in significantly reducing the amount of radiation exposure. Image guided systems such as those taught in U.S. Pat. Nos. 5,772,594 and 6,285,902, incorporated herein by reference, employ a computer and a localizing device to track a drill or drill guide and superimpose a trajectory line on fluoroscopic images. U.S. Pat. No. 6,285,902 describes a system in which, preferably, orthopaedic surgical tools outfitted with infrared LEDs are tracked by an optical localizing device. The poses of these tools are determined and graphic representations of the tools are superimposed on standard intraoperative x-ray images. This allows the surgeon to view, in real time, the position of the tool or tools with respect to an imaged body part or another tool or tools. In the preferred embodiment, a drill guide outfitted with infrared LEDs is tracked and the trajectory of its bore is displayed on the x-ray image of the involved bone. This allows a surgeon to accurately predict the trajectory of a guide pin that passes through the bore of the drill guide. The surgeon can see the path each guide pin will take and can improve the accuracy and speed of insertion of the guide pin with only a minimum number of x-ray images. These systems, however, do not provide help in selecting paths that will lead to the desired triangular pattern with proper spacing between screws. On the other hand, a tracked drill guide with multiple bores may be used, but this requires a large incision and does not leave the surgeon with flexibility in the spacing or orientation (e.g., parallel vs. diverging) of the screws.
Another surgery that is very common in the elderly is total hip replacement. Over 200,000 total hip surgeries are performed each year. This procedure replaces the natural ball and socket joint of the hip, with a metal ball attached to the femur and a plastic lined cup in the socket (acetabulum) of the pelvis. A crucial step in the procedure is the proper alignment of the acetabular cup relative to the pelvic anatomy. Cups that are not properly oriented can lead to dislocation of the new hip and may require reoperation to reorient the cup. However, it can be quite difficult to orient the acetabular cup correctly. Even though the acetabulum is typically fully exposed, and the surgeon has the freedom to place the cup in any orientation, the pelvic landmarks that would give the surgeon the proper frame of reference are not exposed. One such set of landmarks in common usage for defining a pelvic reference frame (albeit for evaluation purposes in post-operative x-rays) are the left and right anterior superior iliac spines (ASIS) and the anterior portion of the pubic bone. During surgery, however, these are not exposed and are thus hidden under sterile draping material and the patient's soft tissue. This problem is even worse with obese patients. Further, the pelvis may be tilted relative to the operating table, making it difficult to use the table or any vertical or horizontal plane as a reliable reference. This leaves the surgeon to orient the acetabular component based on very limited information: the pelvic anatomy exposed in the immediate vicinity of acetabulum, the general body orientation, any visual or palpable evidence of the orientation of the pelvis, and any other clues the surgeon can extract based on experience.
Numerous surgical instruments have been introduced with features designed to assist the surgeon in orienting the acetabular component. U.S. Pat. No. 4,475,549 describes one such instrument, in which handles are oriented such that the cup is properly oriented when the handles are aligned with the patient axis. Again, however, this requires that the surgeon correctly identify the alignment of the patient (i.e., of the patient's pelvis) under the drape and soft tissue.
Image guided techniques have also been applied to the problem of acetabular cup orientation. U.S. Pat. No. 6,002,859 teaches a system for orienting the acetabular cup that is based on a CT scan to preoperatively construct a 3-D model of the pelvis and plan the placement of the acetabular component. During surgery, a registration step is required to correlate the position of the patient's pelvis with the 3-D model. This currently requires the sampling of several points on the patient with a probe and their correlation to the 3-D model. While this system can improve the accuracy, and even the surgical outcome of the total hip procedure, it requires a CT scan which is not currently required, and a potentially time consuming intraoperative registration step.